You will meet Dr Elrath once more (briefly), but not until after you retrieve an item from his cellar. Make sure you explore and loot everywhere there first, since that is easier than trying to get back in afterwards (the door is sealed, so you either have to toss a teleporter stone through a window to get back in (screenshot) or resort to other other means).

The doors inside the house are controlled by something other than keys....
The lanterns act like levers, and if you play a bit you should be able to get to the cellar.

Entering Elrath's house - screenshot with instructions